I myself have (3) three toed box turtles, so let me see if I can help. First of all, an indoor enclosure should be your last result for your turtle. It'll be alright for a baby and a young juvenil, but an adult should definately have an year round outdooor pen. The baby and young juvenil should have an outdorr pen too for the most part, but you have to take MANY measures of securing the pen so no predators can get to them. Minimum indoor encloser will be 5' x 5' an adult if you have to keep it inside due to very cold weather year round. I would lose the cheap way out method of thinking. No matter what, indoor or outdoor enclosure, you'll spend a lot.
No offense, but I can tell you haven't done much research on box turtle care. You need to consult MANY sites for your information. I will answer the questions you asked, but please do more research. Box turtles love fruits, but they need more vegetables. Fruits are their deserts, but they still need them. If a diet of mainly fruit is given, then they will not be open to eating veggies and you'll have a problem. Here's a link about food that I've followed.....
http://www.boxturtlesite.org/diet.html
As far as I know, wwhat water you use doesn't matter. I use water from the hose for my turtles water bowl in the cage and when I soak them. You don't need to worry so much about any vitamin suppliment if you VARY your turtles diet A LOT. However, do some research on the computer, and you'll find a good one. Remember, too much of the suppliment WILL HARM THE TURTLE.
I hope I was of some help.